bash里的here document

不得不再一次说，Advanced bash-scripting guide(a.k.a ABS)这本书太强悍了，对于bash里的Here Document用法，我说见过的其他关于shell/bash编程的书里，都是简单的介绍这个东西是做什么的，基本用法是什么，然后给出一个例子就完成了。
我也一直以为他就是为了便于多行的输入。但是看完ABS对此一整章的介绍，我才发现，其实here document没有那么简单。

这里就不全文翻译了，把几种用法总结一下吧。

简单用法

可以用在简单的命令输出，交互程序里。给出几个例子：

#!/bin/bash # Noninteractive use of 'vi' to edit a file. # Emulates 'sed'. E_BADARGS=85 if [ -z "$1" ] then echo "Usage: `basename $0` filename" exit $E_BADARGS fi TARGETFILE=$1 # Insert 2 lines in file, then save. #--------Begin here document-----------# vi $TARGETFILE <. # Bram Moolenaar points out that this may not work with 'vim' #+ because of possible problems with terminal interaction. exit

进制前导tab输出

这是从曾经遇到的一个问题，在Heredocument里，我为了代码格式好看，采取了tab缩进的方式，但是带来的坏处就是这些tab键也会成为输出，而这并不是我所需要的。因此我需要一个即解决代码格式美观，又不影响输出的办法，原来here document能做到这点，那就是在给出的limit string之前加上一个-符号，比如：

$ ./test.sh
includeing a leading tab
   includeing some spaces

注：它仅仅只是制止前导tab键，对于空格则不受影响。另外要注意的是，如果你的编辑器设置了tab转空格的话，那你会发现以上代码似乎并不是生效。
因为所有的tab都转成4个或者8个或者你设置的数量个空格符号了。

参数替代

here document支持参数和命令替代，可以传递不同的参数给here document文本，从而获得不同的输出，比如：

#!/bin/bash # Another 'cat' here document, using parameter substitution. # Try it with no command-line parameters, ./scriptname # Try it with one command-line parameter, ./scriptname Mortimer # Try it with one two-word quoted command-line parameter, # ./scriptname "Mortimer Jones" CMDLINEPARAM=1 # Expect at least command-line parameter. if [ $# -ge $CMDLINEPARAM ] then NAME=$1 # If more than one command-line param, #+ then just take the first. else NAME="John Doe" # Default, if no command-line parameter. fi RESPONDENT="the author of this fine script" cat <

关闭参数替代

给limit string加上转移或者引号，就可以进制here document的文本参数或者命令替代，这对于生成脚本甚至程序代码提供了帮助。

#!/bin/bash # A 'cat' here-document, but with parameter substitution disabled. NAME="John Doe" RESPONDENT="the author of this fine script" cat < <'Endofmessage' Hello, there, $NAME. Greetings to you, $NAME, from $RESPONDENT. Endofmessage # No parameter substitution when the "limit string" is quoted or escaped. # Either of the following at the head of the here document would have #+ the same effect. # cat <<"Endofmessage" # cat <<\Endofmessage exit

#!/bin/bash # generate-script.sh # Based on an idea by Albert Reiner. OUTFILE=generated.sh # Name of the file to generate. # ----------------------------------------------------------- # 'Here document containing the body of the generated script. ( cat < <'EOF' #!/bin/bash echo "This is a generated shell script." # Note that since we are inside a subshell, #+ we can't access variables in the "outside" script. echo "Generated file will be named: $OUTFILE" # Above line will not work as normally expected #+ because parameter expansion has been disabled. # Instead, the result is literal output. a=7 b=3 let "c = $a * $b" echo "c = $c" exit 0 EOF ) > $OUTFILE # ----------------------------------------------------------- # Quoting the 'limit string' prevents variable expansion #+ within the body of the above 'here document.' # This permits outputting literal strings in the output file. if [ -f "$OUTFILE" ] then chmod 755 $OUTFILE # Make the generated file executable. else echo "Problem in creating file: \"$OUTFILE\"" fi # This method can also be used for generating #+ C programs, Perl programs, Python programs, Makefiles, #+ and the like. exit 0

我们也可以利用here document的输出来设置变量，这其实算是命令替代的一种迂回方式了。

variable=$(cat <

充当函数输入功能

here document也能提供在同一个脚本里一个函数的输入能力。

#!/bin/bash # here-function.sh GetPersonalData () { read firstname read lastname read address read city read state read zipcode } # This certainly looks like an interactive function, but... # Supply input to the above function. GetPersonalData < 或者可以像下面这种匿名功能一样： #!/bin/bash : <

注释代码块

用来快速注释注释一段代码，一般用于调试目的，看下面这个比较长的例子：

#!/bin/bash # commentblock.sh : <<COMMENTBLOCK echo "This line will not echo." This is a comment line missing the "#" prefix. This is another comment line missing the "#" prefix. &*@!!++= The above line will cause no error message, because the Bash interpreter will ignore it. COMMENTBLOCK echo "Exit value of above \"COMMENTBLOCK\" is $?." # 0 # No error shown. echo # The above technique also comes in useful for commenting out #+ a block of working code for debugging purposes. # This saves having to put a "#" at the beginning of each line, #+ then having to go back and delete each "#" later. # Note that the use of of colon, above, is optional. echo "Just before commented-out code block." # The lines of code between the double-dashed lines will not execute. # =================================================================== : <<DEBUGXXX for file in * do cat "$file" done DEBUGXXX # =================================================================== echo "Just after commented-out code block." exit 0 ###################################################################### # Note, however, that if a bracketed variable is contained within #+ the commented-out code block, #+ then this could cause problems. # for example: #/!/bin/bash : <<COMMENTBLOCK echo "This line will not echo." &*@!!++= ${foo_bar_bazz?} $(rm -rf /tmp/foobar/) $(touch my_build_directory/cups/Makefile) COMMENTBLOCK $ sh commented-bad.sh commented-bad.sh: line 3: foo_bar_bazz: parameter null or not set # The remedy for this is to strong-quote the 'COMMENTBLOCK' in line 49, above. : <<'COMMENTBLOCK' # Thank you, Kurt Pfeifle, for pointing this out.

自我文档化

#!/bin/bash # self-document.sh: self-documenting script # Modification of "colm.sh". DOC_REQUEST=70 if [ "$1" = "-h" -o "$1" = "--help" ] # Request help. then echo; echo "Usage: $0 [directory-name]"; echo sed --silent -e '/DOCUMENTATIONXX$/,/^DOCUMENTATIONXX$/p' "$0" | sed -e '/DOCUMENTATIONXX$/d'; exit $DOC_REQUEST; fi : <<DOCUMENTATIONXX List the statistics of a specified directory in tabular format. --------------------------------------------------------------- The command-line parameter gives the directory to be listed. If no directory specified or directory specified cannot be read, then list the current working directory. DOCUMENTATIONXX if [ -z "$1" -o ! -r "$1" ] then directory=. else directory="$1" fi echo "Listing of "$directory":"; echo (printf "PERMISSIONS LINKS OWNER GROUP SIZE MONTH DAY HH:MM PROG-NAME\n" \ ; ls -l "$directory" | sed 1d) | column -t exit 0

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